Dihalides, iron

Of the anhydrous dihalides of iron the iodide is easily prepared from the elements but the others are best obtained by passing HX over heated iron. The white (or pale-green) difluoride has the rutile structure the pale-yellow dichloride the CdCl2 structure (based on cep anions, p. 1212) and the yellow-green dibromide and grey diiodide the Cdl2 structure (based on hep anions, p. 1212), in all of which the metal occupies octahedral sites. All these iron dihalides dissolve in water and form crystalline hydrates which may alternatively be obtained by dissolving metallic iron in the aqueous acid. 

Table 3 Other co-catalysts that have been employed to activate bis(arylimino)pyridine iron dihalides (5)...

When the metallic additive to the intermediate 374 was zinc dihalide (or another Lewis acid, such as aluminum trichloride, iron trichloride or boron trifluoride), a conjugate addition to electrophilic olefins affords 381 . In the case of the lithium-zinc transmetallation, a palladium-catalyzed Negishi cross-coupling reaction with aryl bromides or iodides allowed the preparation of arylated componnds 384 ° in 26-77% yield. In addition, a Sn2 allylation of the mentioned zinc intermediates with reagents of type R CH=CHCH(R )X (X = chlorine, bromine) gave the corresponding compounds 385 in 52-68% yield. ... 

Vinylic halides are less reactive than simple alkenes, but generally afford clean addition products. The addition of HBr to vinyl chloride125 or vinyl bromide61,122,126,127 in the presence of iron(III) chloride or bromide affords primarily the geminal dihalides (equation 78). 

Further restrictions to the scope of the present article concern certain molecules which can in one or more of their canonical forms be represented as carbenes, e.g. carbon monoxide such stable molecules, which do not normally show carbenoid reactivity, will not be considered. Nor will there be any discussion of so-called transition metal-carbene complexes (see, for example, Fischer and Maasbol, 1964 Mills and Redhouse, 1968 Fischer and Riedel, 1968). Carbenes in these complexes appear to be analogous to carbon monoxide in transition-metal carbonyls. Carbenoid reactivity has been observed only in the case of certain iridium (Mango and Dvoretzky, 1966) and iron complexes (Jolly and Pettit, 1966), but detailed examination of the nature of the actual reactive intermediate, that is to say, whether the complexes react as such or first decompose to give free carbenes, has not yet been reported. A chromium-carbene complex has been suggested as a transient intermediate in the reduction of gfem-dihalides by chromium(II) sulphate because of structural effects on the reaction rate and because of the structure of the reaction products, particularly in the presence of unsaturated compounds (Castro and Kray, 1966). The subject of carbene-metal complexes reappears in Section IIIB. 

Diaryl tellurium compounds are converted to diaryl tellurium dihalides through reactions with sulfur tetrafluoride1, triaryl bismuth difluorides2-3, copper(II) halides4,5, iron(III) chloride, and mercury(II) chloride4. 

In the absence of a catalyst, alkynes react very slowly with bromine. Scheme 3.28 particularly when compared to alkenes. When a choice exists, bromine reacts preferentially with an alkene rather than an alkyne. It is possible that radical reactions play a more important role in the addition to alkynes. When the reaction of acetylene with chlorine is catalysed by iron(lll) chloride, the reaction is fast and 1,1,2,2-tetrachloroethane is formed. The uncatalysed addition of a hydrogen halide gives a tram alkenyl halide. Further addition is restricted but can give rise to dihalides. 

In the iron tetracarbonyl dihalide reactions, however, Fe(CF3) complexes were not formed even after months when py or dmf were the solvents, and in CH3CN, CO was displaced by the solvent to yield Fe(CF3) and FeF acetonitrile complexes Fe(CO)4(CF3)2 was not observed. In ether, Fe(CO)4(CF3)2 was isolated from the reaction of Fe(CO)4Br2 with Cd(CF3)3DME after 21 hr at 37 °C. After the product had been dried at — 196°C in high vacuum (52), the yield was 65%. In CH2C12, the reaction of the bromide with Cd(CF3)2(CH3CN)2 resulted in an explosion within minutes of the inception of the experiment. 

It is not always necessary to start with gem-dihalides for the synthesis of aldehydes or ketones. There is a process by which carbonyl compounds can be obtained from monohalides, sometimes in excellent yield for instance, when 3-chlorocyclopentene is stirred vigorously with aqueous sodium dichromate solution at 0°, a chromium complex of 2-cyclopenten-l-one is formed, and this is decomposed to give a 60-68% yield of the ketone if 50% sulfuric acid is dropped in carefully with cooling 513 and 1,2-cyclopentanedione can be obtained in 80% yield by dropwise addition of aqueous iron(m) chloride solution to 2-chlorocyclopentanone in water with rapid stirring at 100°.539... 

The dihalides, RgShHaLg, may be obtained by heating mercury diphenyl with antimony trichloride at a high temperature in an autoclave, or as by-products in the preparation of triarylstibmes by the Fittig reaction. Halogens also add on directly to triarylstibines, gi nng the dihalides, and in the case of chlorine, the following chlorides may replace the free element in the preparation copper, iron, thallium, phosphorus or arsenic chloride. 

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